Normal lung function requires appropriate alveolization during development, and lung injury results in alveolar septal destruction. Repair of damaged lung injury results in alveolar septal destruction. Repair of damaged lung likely involves a recapitulation of normal development to restore alveolar integrity. Alveolar septation involves the interaction of epithelial, endothelial and stromal components under the control of soluble factors, adhesion molecules and micro-environmental signals, through paracrine and autocrine communication, to convert terminal saccules into alveoli. Rat alveolization occurs postnatually and is used as a model to examine factors that regulate this process. Steroid administration during this critical period results in retarded septation, while treatment with trans-retinol rescues both steroid- and injury-induced hypo-alveolization. These findings offer the potential of therapy for clinical diseases such as diffuse alveolar damage and bronchopulmonary dysplasia. While endothelial cells are clearly involved, no information is currently available on the contribution of or factors regulating angiogenesis during alveolization. In order to determine the role of angiogenesis during repair, the angiogenic process during normal development must first be examined. We propose to test the hypothesis that: Normal alveolar formation is dependent on the appropriate tempero-spatial expression on the soluble factor, VEGF, the endothelial cell:cell adhesion molecule, PECAM-1, and the cell:matrix pro-migratory molecule RHAMM, that together promote angiogenesis. To test this hypothesis, we will first: 1. Examine the expression of VEGF & its receptors (VEGFR-1 & -2), PECAM-1 and hyaluronan (HA) and one of its receptors, RHAMM, during normal rat alveolization. We will use northern blot, in situ hybridization, western blot, immunohistochemistry, confocal microscopy and immunogold-labeled electron microscopy for these studies. The second aim of this proposal will: 2. Determine the effect of steroid and/or retinoid treatment on the expression of these molecules during alveolization. In the third aim, using specific blocking antibodies, we will: 3. Determine the role of VEGF, PECAM-1 and RHAMM in angiogenesis and alveolization during development. Collectively these studies will begin to define the molecular regulation of angiogenesis during alveolar formation in normal and steroid-treatment lungs, as well as provide insight into the effects of retinoid treatment on angiogenesis during this critical period of development.